It is known that, as field of view angle characteristics of a liquid-crystal display device, when the screen is viewed obliquely, a reverse phenomenon occurs such that, as a result of the fact that after the luminance temporarily increases with an increase in the gradation, the luminance is decreased, the luminance increases in an area of a lower gradation than in an area of a higher gradation.
In order to improve such field of view angle characteristics, hitherto, a technology in which each of pixels of a liquid-crystal display panel is divided into two subpixels has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2005-316211 published by Japan Patent Office). In this technology, as shown as an example in FIG. 1, a display electrode for one pixel P (a pixel for each of R, G, and B, which are the three primary colors) of a liquid-crystal display panel 50 is divided into electrodes of two subpixels A and B that are driven by a data driver 60 independently of each other.
Then, by setting the driving level (gradation at which the subpixels A and B are driven) of the subpixels A and B to mutually different gradations on the basis of the gradation of the input video signal, the luminance characteristics in a case where the whole pixels P are viewed obliquely are made to approach the luminance characteristics in a case where the whole pixels P are from the front.
In Japanese Unexamined Patent Application Publication No. 2005-316211, which is the above-described document, as such a method of setting drive levels of subpixels, it is described that a gradation conversion table in which the gradation of an input video signal is associated with the output gradation of each subpixel is provided.
Incidentally, in a liquid-crystal display device in which each pixel of a liquid-crystal display panel is divided into two subpixels in the manner described above, in order that the balance of the luminances of R, G, and B when viewed obliquely is improved, there is a case in which it is desirable that the drive level of the subpixel be changed in accordance with whether the pixel is R, G, or B.
FIG. 2 shows an example of such a case. In FIG. 2(a), gradation-luminance characteristics in a case where the drive levels of the subpixels A and B with respect to the input gradation is set as in FIG. 2(b) and the screen is viewed from the front are depicted as GL11. Also, gradation-luminance characteristics in a case where the drive levels of the subpixels A and B are set as in FIG. 2(b) and the screen is viewed obliquely (angle θ) are depicted as GL12.
Here, for example, the gradation values of R, G, and B are assumed to be 128, 96, and 64, respectively. In FIG. 2(a), also, such gradation values of R, G, and B are depicted. In that case, between the gradation-luminance characteristics GL11 and GL12, the ratio of the luminance of R, G, and B when viewed from the front is about 1:2:5, and the ratio of the luminance of R, G, and B when viewed obliquely is about 5:7:10. As a result, when viewed obliquely, since the ratio of the luminance of R becomes small, the red color becomes dark.
In FIG. 2(a), the drive levels of the subpixels A and B with respect to the input gradation are set so as to differ from those of FIG. 2(b) (here, so the gradation values become equal as in FIG. 2(c)), so that gradation-luminance characteristics when the screen is viewed from the angle θ described above are also depicted as GL13. In the gradation-luminance characteristics GL13, the luminance when the gradation value is 128 is higher than that of the gradation-luminance characteristics GL12.
Accordingly, if the drive levels of the subpixels shown in FIG. 2(b) are selected with respect to the pixels of G and B, and the drive level of the subpixel shown in FIG. 2(c) is selected with respect to only the pixel of R, the ratio of the luminance of R when viewed obliquely is increased (the ratio of the luminances of R, G, and B is approached when viewed from the front). As a consequence, it is possible to improve the balance of the luminances of R, G, and B when viewed obliquely.
FIG. 2 shows two sets of gradation values in FIG. 2(b) and FIG. 2(c). If the drive levels of the subpixels for each of the pixels of R, G, and B are selected correspondingly from among three or more types of drive levels shown as an example in FIG. 3, it is possible to even further improve the balance of the luminances of R, G, and B when viewed obliquely.
However, as described in Patent Document 1 described above, in the method in which a gradation conversion table in which input gradations are associated with output gradations so as to allow setting of the drive levels of the subpixels, in order to be able to select a driving level from among a plurality of drive levels, it is necessary to provide a separate gradation conversion table for each driving level. As a result, as shown as an example in FIG. 4, as the number of selectable drive levels is increased, the number of gradation conversion tables for the subpixels A and B increases like TA11 and TB11, TA12 and TB12, . . . TAm and TBm.
Then, in recent years, since the resolution of the gradation has been increasingly improved so as to improve display performance, the amount of data of one individual gradation conversion table is increased. Provision of many such gradation conversion tables with a large amount of data causes the circuit scale of a RAM for storing gradation conversion tables, or the like to increase.
Further, here, the problem in the case that the driving level is selected from among a plurality of drive levels in accordance with whether the pixel is R, G, or B has been described. Still the same problem also occurs even in a case where, for example, the drive level of a subpixel is selected from among a plurality of drive levels on the basis of the type of the input video signal.
In view of the above-described points, it is an object of the present invention to be capable of selecting the drive level of a subpixel with respect to the gradation of an input video signal from among a plurality of drive levels while suppressing an increase in the circuit scale in a liquid-crystal display device in which each pixel of a liquid-crystal display panel is divided into two subpixels.